Microelectronics in spacecraft are vulnerable to degradation and even failure from the radiation dose caused by energetic electrons and ions. It is highly desirable to have radiation sensors onboard the spacecraft to serve as continuous monitors of the radiation exposure. Dosimeters can monitor radiation exposure and should be small devices that can be colocated with the critical electronic systems and economical enough in dollars and spacecraft resources to be routinely included as part of most spacecraft subsystems. A radiation dose is the energy absorbed by an object per unit mass, for example, in ergs per gram for most electronic devices, and particularly, in ergs per gram of silicon.
Dosimeters have been flown on several missions for decades. In some cases, a direct measurement was made of the energy deposited in a test mass of silicon. These measurements can be carried out by integrating the charge created in silicon by space radiation over a satellite mission lifetime. Several test masses were used in each dosimeter, each shielded by a different, hemispherical thickness of aluminum or other spacecraft material. The shield thicknesses can be chosen to represent commonplace shielding of spacecraft systems, so that the dose at those locations could be inferred from the measurement. While the measurement of radiation energy was accomplished through directed methods, the dose at the particular location on the spacecraft was obtained indirectly. However these dosimeters weigh several pounds, consume several watts, and must be mounted on the surface of the spacecraft with a clear 2π view of space. Thus, significant spacecraft resources are disadvantageously required. Additionally, radiation dosimeters require dedicated interfacing to system processors for storing radiation exposure profiles about the spacecraft.
Other flown dosimeter electronic devices follow the degradation of the devices as a function of time and shielding thickness. However, for such a measurement to be useful in a quantitative sense, the sensitivity of a complex electronic component to radiation should be known. Annealing effects, among others, make interpretation difficult. However, the great attraction of such dosimeters is that the dosimeters are very small, require little spacecraft resources, and can be placed inside various spacecraft boxes. The dosimeters could be located directly at the desired spots of necessity on a craft, but made an indirect measurement of a radiation dose. Conventional dosimeters basically are of two types, which may be referred to as Type 1 and Type 2.
The Type 1 dosimeter consists of a box that is mounted on the spacecraft at a single location, with an unimpeded view of the external radiation environment. This sensor uses one or more silicon detectors under selected shield thicknesses representative of typical incidental spacecraft shielding. Thus, a Type 1 dosimeter gives the depth-dose profile of the radiation environment of the host spacecraft in an ideal geometry. The total dose and dose rate data acquired thereby is used with radiation transport codes to determine the radiation dose at points of interest throughout the host spacecraft.
There are several problems associated with the Type 1 dosimeter. First, the dosimeter must be fine-tuned for each host vehicle, and knowledge of the detailed spacecraft design is necessary to complete the dosimeter development. This approach requires significant ray-tracing analysis of the host vehicle design to determine an equivalent thickness of slab-shield or hemispherical-shield geometry to be placed over the detector. This detailed process is prone to error from tolerance build-up in the analysis and from inherent errors in transforming what are typically very complex spacecraft geometries into a simple single thickness of metal. Furthermore, even minor changes in the vehicle design can cause a significant error unless a new ray-tracing analysis is performed. The second problem with the Type 1 dosimeter is the necessary mounting on the spacecraft that requires that each sensor have a clear field-of-view. This is necessary to be able to control the amount of known shielding over the sensors. This constraint puts the dosimeter payload in competition for precious real estate on the spacecraft or requires that the dosimeter settle for some non-ideal spacecraft obstruction to the field-of-view. Finally, radiation transfer calculations are complex and require highly accurate models of the spacecraft of interest in order to obtain accurate results. These calculations are far from trivial or cheap to carry out.
The Type 2 dosimeters make indirect measurements of accumulated dose by measuring the response of an electronic device to ionizing radiation. An example of a Type 2 dosimeter uses a PMOS transistor as the detector element and the associated electronics measure the change in the threshold voltage required to maintain the device at a specified operating point. This Type 2 dosimeter measures the effect of radiation on the gate oxide rather than the silicon, but using the results to infer a silicon dose. This inference is accomplished by comparing flight devices against an extrapolation of prelaunch irradiation measurements to estimate the total dose absorbed by the device. Another dosimeter in this Type 2 class is an EPROM dosimeter where a relationship between memory bit-errors and total dose is used to estimate the dose as in U.S. Pat. No. 5,596,199, issued to McNulty. The main problem with this degradation dosimeter technique is that it is indirect, in that, the devices do not measure radiation dose but the radiation effects upon a specific device. Not all devices degrade in the same way or at the same rate and the understanding of rate and annealing effects become critical. These indirect radiation effects make the interpretation of the device output prone to serious error. A pre-irradiation test of these Type 2 devices is usually performed to establish an operational curve that represents the degradation as a function of the dose received. However, one cannot expose devices to the full range of dose when those same devices are to be positioned in orbit. So, flight devices are typically irradiated at unrealistically low doses on the ground with an extrapolation to predict higher doses actually occurring in orbit.
The most desirable dosimeter is one that makes a direct measurement of the radiation dose and is located directly at the spot of interest. Typically, a radiation dose signal originating in a detector is preamplified and compared to a radiation energy threshold limit, the exceeding of which indicates the reception of a radiation quanta dose in the form of a charge deposit that is then accumulated by integration. Upon reaching this radiation energy threshold limit, integration of the radiation dose signal is a direct measurement of the absorbed radiation energy. A problem with this threshold triggering is that prethreshold charge is not accumulated and represents an error in measurement either by failing to capture the prethreshold charge or by an approximation of the prethreshold triggering energy that is subject to inaccuracies. These and other disadvantages are solved or reduced using the invention.